Mechanical fuel systems, gasoline carburetors, charge forming devices--systems that read air flows with venturies to control the pressure difference across fixed fuel orifices--are constantly plagued by the viscosity flow factors of gasoline at low temperatures and light pressure differences. Gasoline at -20.degree. F. has a viscosity of 4.6, a viscosity of 3.4 at +20.degree. F., and a viscosity of 1.2 at 100.degree. F. Comparing the flow factors at pressure differences of 0.1 PSI and 6.0 PSI: -20.degree. F. yields 0.228 and 0.865, respectively; +20.degree. F. yields 0.300 and 0.900; and 100.degree. F. yields 0.900 and 0.995. 0.1 PSI is the equivalent of 2.8 ins. H.sub.2 O, and 2.8 ins. pressure difference across a venturi has an air velocity of 100 feet per second or 75 miles per hour, which is about 17% of max air. Comparing these metering pressure differences with those used with the ported injection systems of about 25 PSI, the factors become 0.945 at -20.degree. F, 0.96 at +20.degree. F. and 1.00 at 100.degree. F. These figures pretty well describe the difficulty of metering with a simple carburetor.